Reverse engineering HomeKit QR Code decoding/encoding

HomeKitQR.swift

import Foundation

func setupCodeFrom(uri: String) -> String {
    let startIndex = uri.index(uri.startIndex, offsetBy: "X-HM://".count)
    let endIndex = uri.index(uri.endIndex, offsetBy: -4)
    var actualString = uri[startIndex..<endIndex]
    
    let number = UInt(actualString, radix: 36)!
    
    var code = String(format: "%08u", number & 0x7ffffff)
    code.insert("-", at: code.index(code.startIndex, offsetBy: 3))
    code.insert("-", at: code.index(code.startIndex, offsetBy: 6))
    
    let category = HKCategory(rawValue: number >> 0x1f & 0xff)
    let flags = number >> 0x1b & 0xf
    let reserved = number >> 0x27 & 15
    let version = number >> 0x2b & 7
    return code
}

func setupCodeToURI(code: UInt) -> String {
    let version: UInt = 0
    let flags: UInt = 2
    let reserved: UInt = 0
    let category: HKCategory = .bridge
    
    let resAndVer = reserved & 0xf | version << 0x4 & 0x70
    let catResAndVer = category.rawValue & 0xff | (resAndVer & 0xff) << 0x8
    let other = flags & 0xf & 0xff | catResAndVer << 0x4
    let resnum = other << 0x1b | code & 0x7ffffff
    
    var outString = String(resnum, radix: 36, uppercase: true)
    while outString.count < 9 {
        outString.insert("0", at: outString.startIndex)
    }
    
    return outString
}

setupCodeFrom(uri: "X-HM://0009N5SP0JJ3K")
setupCodeToURI(code: 23748153)

enum HKCategory: UInt {
    case other = 1
    case bridge = 2
    case fan = 3
    case garage = 4
    case lightbulb = 5
    case doorLock = 6
    case outlet = 7
    case `switch` = 8
    case thermostat = 9
    case sensor = 10
    case securitySystem = 11
    case door = 12
    case window = 13
    case windowCovering = 14
    case programmableSwitch = 15
    case rangeExtender = 16
    case ipCamera = 17
    case videoDoorBell = 18
    case airPurifier = 19
    case reserved = 20
}

Thanks for this gist. I tested it out on a new accessory that I unboxed today. This accessory's URI is much longer:

X-HM://0EBML5MNUXZEI07UG9503PZKTR0XK3K

This causes setupCodeFrom to raise an exception. I fixed it by ignoring the extra characters, changing line 5 to this:

let endIndex = uri.index(startIndex, offsetBy: 9)

(I assume the extra characters at the end are some sort of vendor-specific parameters used during setup.)